This research project will investigate the cyclic nucleotide phosphodiesterase enzymes of mammalian tissues. Using initially the three rat liver enzymes resolved by ion exchange chromatography, these proteins will be further purified to homogeneity so that they may be studied with respect to subunit structure and related allosteric properties. Additional experiments will be carried out using chemical labelling and crosslinking methods to localize the phosphodiesterase with respect to cell components and to the other enzymes of cyclic nucleotide metabolism. Correlations will be sought between the properties and location of the phosphodiesterase activities and their role in physiological regulation. The latter aspect will emphasize the potential role of these enzymes in the primary mechanism of insulin action.